The SRAM website says the X.9 rear derailleur is 8/9 speed, but the X.9 trigger appears to be 9 speed only. The X.7 trigger is listed as 9/8 so maybe some mix and match would work. No idea about the 8-speed part availability though. Additionally, you'll likely have 9-speed chain, rings, and front derailleur too...

On the other side of things, is really it worth that hassle to save a few grams on a full suspension bike anyway? I'd imagine there are easier places to lose weight.